I would take this rumour lightly as it sounds just like that, a rumour. Sounds like someone or the Japanese tabloids or whatever just came up with this "conclusion" because she hasn't been around lately.

From the interviews, she seemed to be enjoying them very much. She was getting much better at interviews and seemed even more outspoken than she is for the Japanese press. And not liking small areas to sing in sounds like a very "diva" thing and we all know that Utada is no diva. She's very down-to-earth and humble and that's why we love her.